Given that just over a million gallons are leaking into the Gulf per day, according to MacDonald's calculations, the spill will shortly top the Exxon Valdez's estimated 11-million-gallon spill. It is almost certain to cost more than the Exxon spill, which cost $3.5 billion for cleanup and another $5 billion worth of lawsuits and other settlements.

The environmental whistleblowers at SkyTruth, which debunked earlier lowball estimates from the government and BP, said the spill will top the Exxon spill by the end of the day Saturday. Federal point man and Coast Guard Admiral Thad Allen didn't dispute the calculations of MacDonald or SkyTruth, but said "any exact estimate is probably impossible at this point."